An individual battery cell has a rather low voltage, typically in the range of 1 to 2.2 volts. This low voltage is quite suitable for some purposes, such as small flashlights, watches, handheld calculators and personal radios. However, a single cell is inadequate for uses which have large power requirements, such as forklifts, golf carts, electric vehicles, electrically started vehicles, and back-up power supplies. Therefore, many battery-powered devices require a higher voltage. For example, automobiles typically require 6 or 12 volts, some diesel powered vehicles require 24 volts, uninterruptable power supply (UPS) systems require 120 or 240 volts, and some other systems require even higher voltages. The battery cells are connected in series to achieve these higher voltages. For example, a nominal 12 volt automobile battery will have six cells connected in series, each cell having a charged voltage of approximately 2.2 volts. Likewise, twelve cells are connected in series to provide a nominal 24 volt supply. To achieve even higher voltages cells are typically connected in series to form batteries, as described above, and the batteries are connected in series to form battery packs. For example, ten 12 volt batteries are connected in series to provide a nominal 120 volt supply. The current handling capability of a single cell or battery is limited by practical considerations so cells, batteries, and battery packs are connected in parallel to achieve higher currents.
One example of a UPS battery configuration is made by Exide Corporation, and has a full charge rating of 132 volts, has 60 batteries rated at 1000 amp hours each, and can accept a charging current of 200 amperes. UPS systems are typically installed at hospitals, television and radio stations, telephone switching stations, and other places where the noninterruption of electrical power is critical.
If battery cells could be made absolutely identical to each other and subjected to identical conditions then series-connected cells would have the same states of charge throughout their lifetimes. However, battery cells cannot be made to be absolutely identical to each other so some cells discharge, charge and age faster than other cells. As a result, at some point the different cells may have such different states of charge that one or more cells may be fully charged but other cells may have minimal or no charge. When a cell finally reaches the point that it is discharged but the other cells are still at least partially charged, further use of the series-connected cells will cause the discharged cell to be subjected to a reverse polarity voltage, which can cause further deterioration of that cell, overheating, gassing, or even an explosion.
Likewise, batteries cannot be manufactured to be absolutely identical. Furthermore, whereas the cells in a battery may have been manufactured from the same materials, activated at approximately the same time, and subjected to approximately the same temperature conditions so that there is some degree of match between the individual cells, the same cannot be said for different batteries. That is, one 12 volt battery may be a year older than another 12 volt battery and may have been subjected to more or fewer charge/discharge cycles, more or fewer deep discharge cycles, higher or lower temperature extremes, etc. Also, batteries which are on the outside of a battery pack have better ventilation and may be cooler than batteries which are on the inside of the battery pack and have poor or no ventilation. However, batteries on the outside of the battery pack may also be subjected to greater and more rapid extremes in temperature than the batteries which are on the inside of the battery pack and are therefore somewhat insulated from the surrounding environment.
Therefore, it is more likely than not that the temperature, the internal impedance, and the state of charge will be different from battery to battery in a battery pack and will be exaggerated as the batteries undergo aging, temperature cycling, and charging/discharging cycles. Thus, at some point, one of the batteries will reach a state of zero charge when others of the batteries still have substantial charges and further discharging of the battery pack will cause the battery with zero charge to be subjected to a reverse polarity voltage, with the same consequences for that battery as described above for an individual cell which is reverse charged.
Equalization is the process whereby all of the batteries are brought to the same state of charge. Equalization is very important because it prevents the application of a reverse polarity voltage to a battery. Also, the internal impedance will be different from one battery to another. The internal impedance depends upon the state of charge of the battery, the temperature of the battery, the amount of electrolyte present, the amount of water in the electrolyte, and the state (deterioration) of the electrodes. A good battery will have a lower impedance when fully charged and a higher impedance when fully discharged. The more that the charging voltage exceeds the battery voltage, the more the current that will be forced into the battery. If the amount of current forced into the battery exceeds the current that the battery can use for charging then the excess current will cause electrolysis of the battery water, gassing, and heating of the battery. Therefore, when a charging current is applied to a battery pack greater heating will occur in a more fully charged battery than the heating in a lesser-charged battery. The states of charge between different batteries may be somewhat equalized by continuing to apply a charge to the battery pack even though some of the batteries have already been completely charged. However, gassing as well as overheating of these more fully charged batteries may occur. Furthermore, if high a current pulse charging technique is used then the application of a large charging current pulse to a fully charged battery may cause damage to or catastrophic failure of the battery.
At 90% of full charge, a battery will not readily accept a high charging rate. Therefore, if the charging current is set so as to rapidly charge the weakest battery, the charging current will be too high for a more fully charged battery and damage can be done to the more fully charged battery. However, if the charging current is reduced to prevent damage to the more fully charged battery then the equalization process will take a much longer time and will not be finished at the same time that the charging process is finished. For example, if each battery in a battery pack has a full-charge rating of 12 volts and 200 ampere-hours, all batteries but one are fully charged, and this one battery has a state of charge of only 90% of full charge, then 20 ampere-hours of charging current must be applied to that battery to bring it to a full charge. To accomplish this, a 20 amp charge could be applied for 1 hour, or a 40 amp charge could be applied for 30 minutes, or a 160 amp charge applied for 7.5 minutes, etc. However, the fully charged batteries may not accept the 160 amp charging current, or even the 40 amp charging current, without overheating, gassing, or damage. Therefore, to avoid damage to the fully charged batteries during the equalization process, the charging current must be limited to 20 amperes, or less, and the charging time must be extended to 1 hour, or more, to add enough charge to the lesser charged battery to bring it to the same full charge level as the other batteries.
Even though equalization is important, most persons regard batteries as "install and forget" items, so the equalization process is rarely performed on a regular basis in actual practice.
Therefore, there is a need for a method and an apparatus for automatically, continuously, and rapidly equalizing the state of charge among a plurality of series-connected cells or batteries during a charging process in a manner which will not cause damage to the cells or batteries.